Active matrix liquid crystal display panel

ABSTRACT

An active matrix liquid crystal display panel by which a good display characteristic can be obtained without suffering from gradation reversal over a wide visibility angle range. A liquid crystal layer  4  is formed such that the thickness-thereof varies in accordance with transmission wavelengths of color layers  6, 7  and  8  so that a very good display which does not exhibit any coloring in whichever direction it is viewed may be obtained.  
     An active matrix substrate A includes a plurality of opposing electrodes  2,  a plurality of pixel electrodes  3  parallel to the opposing electrodes  2,  a thin film transistor, and an orientation film  23  all formed on a glass substrate  10.  A color filter substrate C includes an orientation film  56  provided on one surface of another glass substrate  10  and an optical compensation layer  35  provided on the other surface of the glass substrate  10  and formed from a plastic film. The two substrates are disposed such that the orientation films thereof oppose each other, and polarization plates  34  and  5  are disposed on the outer sides of the two substrates, and a liquid crystal layer  4  having a positive refractive index anisotropy is provided between the orientation films 23. The optical compensation layer  35  has a negative one axial refractive index anisotropy and can cancel a retardation produced in the liquid crystal layer  4  thereby to suppress white floating of a black display portion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an active matrix liquid crystal display panelof the structure wherein liquid crystal is held between transparentinsulating substrates.

2. Description of the Related Art

An active matrix liquid crystal display panel (hereinafter referred toas AMLCD) wherein a thin film field effect transistor (hereinafterreferred to as TFT) is used as a switching element for a pixel has ahigh picture quality and is utilized widely as a display device for aportable computer or a monitor for a desk top computer of the spacesaving type.

In recent years, in order to achieve a high quality of a liquid crystaldisplay, a display method called in-plane switching mode which makes useof a transverse electric field in order to improve the visibility anglecharacteristic has been proposed (for example, Asia Display '95) (PriorArt 1).

According to the display method, a pixel electrode and an opposingelectrode are formed in parallel to each other, and a voltage is appliedbetween the pixel electrode and the opposing electrode to form aparallel electric field in a plane of a liquid crystal layer to vary thedirection of the director of the liquid crystal thereby to control theamount of transmission light therethrough.

In the liquid crystal display system described above, since the directormoves only in a direction substantially parallel to and in the plane ofthe liquid crystal layer, such a problem that, as the director rises outof the plane of the liquid crystal layer as in the TN (Twisted Nematic)mode, the relationship between the transmission light amount and theapplied voltage exhibits a large difference whether the liquid crystallayer is viewed from the direction of the director or from the directionof a normal to the liquid crystal layer does not occur, and a displayimage which looks in a similar manner from whichever direction it isviewed can be obtained over a very wide visual angle.

FIG. 1 is a view showing a liquid crystal display system which is drivenby a transverse electric field and exhibits a good displaycharacteristic.

For the display system described above, several systems have beenproposed depending upon the initial orientation condition of the liquidcrystal layer and the manner of setting of polarizing plates. Of thosesystems, such a system as shown in FIG. 1 wherein a liquid layer isinjected in the same direction on both substrates and, in the initialorientation condition, the directors are oriented uniformly in thisdirection while one of two polarizing plates between which thesubstrates are held and which form a cross nicol is oriented to thedirection of the directors in the initial condition so that, when novoltage is applied, a black display is obtained, but when a voltage isapplied, the directions of the directors are turned to obtain a whitedisplay, is considered advantageous in that the black level can stablebe made low.

In the display mode of the display system described above, thetransmission factor T of light coming in from the front is given inaccordance with the turning angle φ of the directors based on thefollowing expression:T=sin²(2φ)·sin²(π·Δnd _(eff)/λ)   (1)

where d_(eff) is the effective value of the liquid crystal layerthickness which undergoes turning deformation when the liquid crystaldirectors are twist deformed while they are large at a central portionand are fixed at interfaces of the liquid crystal with the substrates,and is smaller than the actual liquid crystal layer thickness.

It has been experimentally confirmed that, for example, where a liquidcrystal cell of 4.5 μm thick is formed and liquid crystal having adielectric constant anisotropy Δn=0.067 is injected in the liquidcrystal cell, if a transverse electric field is applied so as to inducea deformation corresponding to φ=45 degrees, the transmission factorexhibits a wavelength dependency as seen from the expression (1) and hasa maximum value substantially at λ=550 nm. Conversely, from this, it isesteemed that d_(eff)=4.1 μm using the expression (1), and thetransmission factor for any other wavelength substantially coincideswith a value obtained by substituting d_(eff)=4.1 μm into the expression(1).

In this instance, between the representative wavelength 460 nm selectedby a color filter of blue and the representative wavelength 610 nmselected by another color filter of red, the transmission factor givenby the expression (1) varies within a range less than 10% the highestvalue thereof. However, even if a special process is not performed, asignificantly coloring image does not look when the liquid crystal cellis viewed from the front. Where a higher color purity is required,transmission lights from the color filters of R, G and B can be balancedwell by adjusting the transmission factors of the color filters or thespectrum of back light.

It is examined here that, when a transverse electric field is applied toturn the directors approximately by 45 degrees to provide a whitedisplay, a substrate is viewed obliquely from a direction perpendicularto the turned directors.

FIGS. 2(a) and 2(b) are views illustrating transmission of light throughliquid crystal when it comes in obliquely, and wherein FIG. 2(a) is aview as viewed from an oblique direction with respect to a substrate andFIG. 2(b) is a view as viewed from a parallel direction to thesubstrate.

While the transmission factor of light passing obliquely through aliquid crystal cell is not precisely represented by the expression (1),it is essentially same in that the light passes through a cross nicol asa retardation is produced between an ordinary ray and an extraordinaryray when it passes through the liquid crystal. Accordingly,f=sin²(π·ΦnL/λ)   (2)

wherein d_(eff) of the second factor of the right side of the expression(1) is replaced with the optical path length L when a ray passes throughthe effectively turned liquid crystal layer, makes an important factorwhich dominates the intensity of the transmission light.

When the liquid crystal cell is viewed from the front, with green lightcorresponding to λ=550 nm, the transmission factor spectrum has amaximum value, and consequently,π·Δnd _(eff)/λ=π/2   (3)and the factor f in expression (2) is 1.

As seen from FIGS. 2(a) and 2 b), when the liquid crystal cell is viewedfrom a direction which is perpendicular to the directors and oblique tothe substrate, the refractive index anisotropy Δn felt with transmissionlight is the difference in length between the major axis and the minoraxis of an ellipse which corresponds to a section of a refractive indexellipsoid of revolution having a major axis in the direction of thedirectors of the liquid crystal when the refractive index ellipsoid ofrevolution is cut along a wave front of the ray. In this instance, sincethe wave front includes the major axis of the ellipsoid of revolution,the refractive index anisotropy Δn felt with the light is fixedirrespective of the inclination angle θ from the direction of a normalto the substrate. Accordingly, as the inclination angle θ increases,π·ΔnL/λ gradually increases from π/2, while the factor f given by theexpression (2) decreases and, reflecting this, also the transmissionfactor T decreases.

With light of red corresponding to λ=610 nm,π·Δnd _(eff)/λ<π/2   (4)on the front, and the factor f is smaller than 1. From the same reasonas in the case of λ=550 nm, as θ increases, π·ΔnL/λ increases, and afterit becomes equal to π/2, it further increases exceeding π/2. In responseto the increase, also the factor f becomes equal to 1 once, andthereafter decrease gradually. Consequently, also the transmissionfactor T reflects this and increases once and then decreases gradually.

On the other hand, with light of blue corresponding to λ=460 nm,π·Δnd _(eff)/λ>π/2   (5)on the front, and the factor f is smaller than 1. From the same reasonas in the case of λ=550 nm, as θ increases, π·ΔnL/λ increases and isspaced farther from π/2. Consequently, f decreases farther from 1. Sincethe rate of increase of f when the optical path length L increases isgiven byδf/δL=(π·Δn/λ)·sin(2π·ΔnL/λ)   (6)as π·ΔnL/λ increases exceeding π/2, f decreases suddenly. Accordingly,it can be said that the decrease of f where λ=460 nm is more sudden thanthat when λ=550 nm, and also the transmission factor T decreasessuddenly.

From the foregoing, since, as θ increases, blue light decreases mostsuddenly and green light decreases comparatively moderately whereas redlight first increases and then decreases, although white light looks onthe front, as θ increases, the light gradually appears coloring withred.

This can be confirmed more quantitatively by a simulation which isperformed taking a deformation and an optical anisotropy of liquidcrystal into consideration.

FIG. 3 is a diagram illustrating a relationship between the inclinationangle and the transmission factor when, in order to display white, lightcomes into a substrate from a direction perpendicular to the liquidcrystal directors and oblique to the substrate. It is to be noted thatthe axis of abscissa indicates the inclination angle θ and the axis ofordinate indicates a result of calculation of the transmission factornormalized with the transmission factor on the front.

As seen from FIG. 3, as θ increases, the transmission factor generallydecreases, and above all, it can be seen that the transmission factorfor blue decreases most rapidly.

FIG. 4 is a diagram illustrating a relationship between the inclinationangle and the transmission factor when, in order to display white, lightcomes into a substrate from a direction same as the direction of theliquid crystal directors and oblique to the substrate.

As seen in FIG. 4, when the line of sight is gradually inclined to thesame direction as that of the directors in a white display, if a similarsimulation is performed, it can be seen that red light converselyexhibits the most significant attenuation.

The phenomena described above occur quite similarly with an actual colorliquid crystal display panel on which color filters are provided. Infact, it has been confirmed that, when a color liquid crystal panelproduced in the same conditions as those of the liquid crystal celldescribed above is viewed from an oblique direction, it looks coloring.

As described above, with an active matrix liquid crystal displayapparatus which is constructed using a transverse electric field,although a good display characteristic is obtained over an angle ofvisibility wider than that of a conventional TN mode, when viewed froman oblique direction, depending upon the direction, a display imagelooks coloring significantly. If such coloring occurs, then when imagedata of full colors are to be displayed, the image of the originalpicture is deteriorated remarkably.

On the other hand, methods of forming, in a liquid crystal display panelhaving color filters, liquid layers for the colors of the color filterswith different layer thicknesses are disclosed in Japanese PatentLaid-Open Application No. Showa 60-159831 (Prior Art 2) and JapanesePatent Laid-Open Application No. Showa 60-159823 (Prior Art 3). Themethods propose a display system wherein liquid crystal is held betweentwo glass substrates and a voltage is applied between transparentelectrodes on the opposite sides of the liquid crystal to vary thealignment of the liquid crystal layer, above all, of a liquid crystaldisplay apparatus of the twisted nematic (TN) mode, and besides relatesto a method of optimizing the characteristic when the liquid crystaldisplay apparatus is viewed from the front. Those methods are quitedifferent in structure, purpose and principle from the present inventionwhich has been made to suppress coloring which occurs upon oblique lightincidence in a transverse electric field display system which has apicture quality much higher than that of the TN system as hereinafterdescribed.

Different methods are proposed in Japanese Patent Laid-Open ApplicationNo. Heisei 1-277283 (Prior Art 4) and Japanese Patent Laid-OpenApplication No. Heisei 6-34777 (Prior Art 5) wherein the thickness of aliquid crystal layer is optimized for individual colors in order toimprove the characteristic on the front in simple matrix driving.Similarly, however, the methods are essentially different from thepresent invention.

Further different techniques are proposed in Japanese Patent Laid-OpenApplication No. Showa 60-159827 (Prior Art 6), Japanese Patent Laid-OpenApplication No. Heisei 2-211423 (Prior Art 7) and Japanese PatentLaid-Open Application No. Heisei 7-104303 (Prior Art 8) wherein liquidcrystal layers are formed with different thicknesses for the colors ofcolor filters. However, they relate to a structure and a productionmethod proposed to optimize the front characteristic of the TN mode andare essentially different from the present invention.

As described above, with an active matrix liquid crystal displayapparatus which is constructed using a transverse electric field, whilea good display characteristic is obtained over a wider angle ofvisibility than that of the conventional TN system, there is a problemin that, when viewed from an oblique direction, significant coloringappears depending upon the direction, and consequently, when image datasuch as, for example, a photograph are to be handled, the image of theoriginal picture is deteriorated very much.

Again, in recent years, in order to achieve a higher quality of a liquiddisplay, a displaying method called in-plane switching mode (hereinafterreferred to simply as “IPS”) which makes use of a transverse electricfield in order to improve the visibility angle characteristic has beenproposed. An example was published in “Asia Display '95 “held in Oct. 10to 18, 1995 and is disclosed in “Principles and Characteristics ofElectro-Optical Behaviour with In-Plane Switching Mode”, a Collection ofDrafts for the Asia Display '95. The liquid crystal panel disclosed isconstructed such that, as shown in FIG. 5, a linear pixel electrode 71and a linear opposing electrode 72 are formed in parallel to each otheron one of a pair of substrates 70 between which a liquid crystal layeris held, but no electrode is formed on the other substrate. A pair ofpolarizing plates 73 and 74 are formed on the outer sides of thesubstrates 70 and have polarization axes 75 and 76 extendingperpendicularly to each other. In other words, the polarizing plates 73and 74 have a positional relationship of a cross nicol to each other. Avoltage is applied between the pixel electrode 71 and the opposingelectrode 72 to produce a transverse electric field 77 parallel to theplane of the liquid crystal layer, whereupon the directions of thedirectors of liquid crystal molecules are varied from an initialorientation direction 78 thereby to control transmission light throughthe liquid crystal layer.

In the twisted nematic mode (hereinafter referred to simply as “TN”),since liquid crystal molecules rise three-dimensionally from the planeof the liquid crystal layer, the manner in which the liquid crystallayer looks is different whether it is viewed in a direction parallel tothe directors of rising liquid crystal molecules or in another directionnormal to the liquid crystal layer. Further, there is a problem in that,when the liquid crystal display panel is viewed from an obliquedirection, the relationship between the applied voltage and thetransmission light amount is different very much. More particularly, asseen from a voltage-transmission factor characteristic illustrated as anexample in FIG. 6, where a liquid crystal display panel of the TN modeis viewed from the front, the characteristic makes a monotonouslydecreasing curve wherein the transmission factor decreases as theapplied voltage increases after it exceeds 2 V. However, where theliquid crystal display panel of the TN mode is viewed from an obliquedirection, the characteristic makes such a complicated curve havingextremal values that, as the applied voltage increases, the transmissionfactor decreases once until it comes to 0 at the voltage of 2 V, but asthe voltage thereafter increases, the transmission factor increasesuntil it decreases again after the voltage exceeds approximately 3 V.Accordingly, if the driving voltage is set based on thevoltage-transmission factor characteristic when the liquid crystaldisplay panel is viewed from the front, then when the liquid crystaldisplay panel is viewed from an oblique direction, there is thepossibility that gradation reversal may occur such that a whitedisplaying portion looks black or a black displaying portion becomeswhitish. After all, normally the display of the liquid crystal displaypanel of the TN mode is visually observed correctly and can be used onlywithin the range of the angle of visibility of 40 degrees in theleftward and rightward directions, 15 degrees in the upward directionand 5 degrees in the downward direction. Naturally, the upward,downward, leftward and rightward directions can be modified by theinstallation of the liquid crystal display panel.

On the other hand, the in-plane switch (IPS) system is advantageous inthat, since liquid crystal molecules move only in directionssubstantially parallel to the plane of the liquid crystal layer(two-dimensionally), a substantially similar image can be obtained asviewed from an angle of visibility wider than that of the TN system.Particularly, the IPS system can be used within the range of an angle ofvisibility of 40 degrees in the upward, downward, leftward and rightwarddirections.

As apparatus of the IPS system, various liquid crystal display panelshave been proposed which have various constructions depending upon theinitial orientation condition of the liquid crystal layer and the mannerof setting of polarizing plates. In the example of FIG. 5 describedabove, the liquid crystal layer is processed by interface orientationprocessing in the same direction for the two substrates and thepolarization axis of one of the two polarizing plates extends inparallel to the orientation direction. This liquid crystal display panelallows a stabilized black display since, in the initial orientationcondition, the directors of liquid crystal molecules are orienteduniformly in the direction of the interface orientation processing andblack is displayed when no voltage is applied, but when a voltage isapplied, the directors are turned so that white is displayed.

As described above, with an active matrix liquid crystal display panelof the IPS system which makes use of a transverse electric field, a gooddisplay characteristic can be obtained over an angle of visibility widerthan that of the conventional TN system. However, also the active matrixliquid crystal display panel of the IPS sometimes suffers from gradationreversal depending upon the angle at which the active matrix liquidcrystal display panel is viewed. Where gradation reversal occurs in thismanner, there is a problem that, if an image principally of a blackcolor such as hair of a human being is displayed, then a good imagecannot be obtained when it is viewed from an oblique direction to theactive matrix liquid crystal display panel.

This problem is described in more detail below. First, the transmissionfactor where the liquid crystal layer is omitted and only two polarizingplates are disposed in a positional relationship of a cross nicol toeach other. It is to be noted that, of the two polarizing plates, thatone which is disposed on the light incoming side is a polarizer, and theother one which is disposed on the light outgoing side is an analyzer.

In FIG. 7, the unit vector in the absorption axis direction of thepolarizer is represented by e₁, the unit vector in the absorption axisdirection of the analyzer by e₂, and the unit vector in the substratenormal direction by e₃. Those unit vectors extend perpendicularly toeach other. The unit vector in the direction of a ray when it passesthrough the polarizer is represented by k. Where the angle between thevector k and the substrate normal line is represented by a zenithalangle α and the angle between a projection of the vector k on the planeof a substrate and the vector e₁ is represented by an azimuth φ, thevector k is represented ask=sin α cos φ·e ₁+sin α·sin φ·e ₂+cos α·e ₃   (7)

Light when it passes through the polarizer can be considered to becomposed of a polarized light component of the (e₁×k) direction andanother polarized light component of the ((e₁×k)×k) direction. It is tobe noted that the symbol “x” between vectors represents the product ofthe vectors. Since the former is normal to the absorption axis e₁,theoretically it is not absorbed. On the other hand, the latter isabsorbed by the polarizer. If the product of the absorption coefficientand the film thickness of the polarizer is sufficiently large, then thelatter polarized light component is 0 after the light passes through thepolarizer.

The refractive indices of the two polarizing plates (polarizer andanalyzer) are substantially equal to each other and the directions ofthe ray when it passes through the analyzer is equal to k, when the raypasses through the analyzer, the light is separated into a polarizedlight component of the (e₂×k) direction and another polarized lightcomponent of the ((e₂×k)×k) direction. The latter polarized lightcomponent is absorbed substantially completely during passage throughthe analyzer while only the former polarized light component remains.Accordingly, if the influence of reflection at the surface of the glassand so forth is ignored, then the transmission factor T is representedas $\begin{matrix}{T = \left\{ {\frac{1}{\sqrt{2}} \cdot \frac{e_{1} \times k}{{e_{1} \times k}} \cdot \frac{e_{2} \times k}{{e_{2} \times k}}} \right\}^{2}} & (8)\end{matrix}$

By representing the expression (8) using α and φ, $\begin{matrix}{T = {\frac{1}{2} \cdot \frac{\sin^{4}{\alpha \cdot \sin^{2}}{\phi \cdot \cos^{2}}\phi}{{\sin^{4}{\alpha \cdot \sin^{2}}{\phi \cdot \cos^{2}}\phi} + {\cos^{2}\alpha}}}} & (9)\end{matrix}$is obtained.

When light comes in from an azimuth equal to the direction of theabsorption axis of one of the polarizing plates such as where theazimuth φ is 0 degree or 90 degrees, the transmission factor T is 0 fromthe expression (8). In other words, similarly to the case wherein lightcomes in from the front, the light does not pass due to the action ofthe polarizing plates which are at the positions of a cross nicol.

On the other hand, where the azimuth φ=45 degrees, that is, where theazimuth φ defines 45 degrees with respect to each of the absorption axesof the two polarizing plates, as the zenithal angle α increases, thetransmission factor increases. Where the refractive index of thepolarizer is 1.5, since the refractive index of the air is approximatelyequal to 1, the highest value of sin α is approximately 1/1.5. If thisis substituted into the expression (9) to calculate it, the resultingtransmission factor is approximately 7%. Actually, however, sincereflection occurs at the interface between each of the polarizing platesand the air due to the difference in refractive index between them, if asimulation is performed taking the reflection into consideration, thenthe relationship between the inclination angle (zenithal angle) α of theray in the air with respect to a normal to the substrate and thetransmission factor is such as indicated by a curve 1 of FIG. 8.

Next, another case is described wherein liquid crystal having a positivedielectric constant anisotropy and having a refractive index anisotropywith n_(o)=1.45 and Δn=0.067 is held between two polarizing plates suchthat the directors are oriented in the same direction (Δ=90 degrees andφ=0 degree) as that of the absorption axis of the analyzer. Light havingpassed through the polarizer advances, in the liquid crystal, in adirection a little different from the direction of the light in thepolarizer. As a result, the linearly polarized light polarized uniformlywhen it passes through the polarizer becomes elliptically polarizedlight after it passes through the liquid crystal. Consequently, thetransmission factor is different from that where the liquid crystal isabsent. The relationship between the zenithal angle α and thetransmission factor when light comes in from the direction of theazimuth φ=45 degrees is indicated by a curve 2 in FIG. 8. In thisinstance, the transmission factor is rather higher than that (curve 1)where only the polarizing plates of a cross nicol are arranged while noliquid crystal layer is present.

On each substrate interface, the liquid crystal directors do not extendcompletely parallel to the plane of the substrate but normally rise byapproximately 1 to 10 degrees with respect to the plane of thesubstrate. This angle is a pretilt angle. Usually, since, in order toorient liquid crystal with a higher degree of stability, interfaceorientation processing such as rubbing is performed such that theorientation directions of liquid crystal molecules may extend inparallel to each other in the proximity of each interface, the liquidcrystal molecules are inclined by a fixed angle with respect to theplane of the substrate substantially in all regions. Where anorientation film for industrial use which is high in stability isemployed, generally the pretilt angle is approximately 3 degrees.

The relationship between the zenithal angle a and the transmissionfactor where the pretilt angle is 3 degrees and light comes in from thedirection of the azimuth φ=45 degrees is such as indicated by a curve 3in FIG. 8. Further, the relationship between the zenithal angle α andthe transmission factor where the pretilt angle is −3 degrees and lightcomes in from the direction of the azimuth φ=45 degrees is such asindicated by a curve 4 in FIG. 8. It is to be noted that the pretiltangle when the liquid crystal rises in the same direction as the vectore₁ is taken as positive, and the pretilt angle when the liquid crystalrises in the opposite direction to the vector e₁ is taken as negative.Particularly where the liquid crystal rises in the same direction as thevector e₁ (where the pretilt angle is positive), the transmission factorhas a value approximately twice that in the case where only thepolarizing plates are present (no liquid crystal is present).

Since the curves 1 to 4 of FIG. 8 exhibit comparison among black displayconditions when no electric field is applied to the liquid crystal asdescribed above, preferably the transmission factor is as low aspossible. However, the curve 3 has a very high transmission factorcomparing with the curves 1, 2 and 4. Therefore, the case of the curve3, that is, the case wherein the pretilt angle is 3 degrees, isdescribed in more detail.

While description has been given above of the case wherein no electricfield is applied to the liquid crystal, if a transverse electric fieldis applied to the electric field to turn the directors in the plane ofthe liquid crystal layer, then the transmission factor increases.According to a simulation by calculation, the transmission factor whenthe potential difference between a pixel electrode and a commonelectrode is 3 V is approximately 2.4%, and the transmission factor whenthe potential difference is 3.5 V is approximately 6.3%. FIG. 9 showsgraphs obtained by plotting results of calculation of the transmissionfactor variation when the zenithal angle α is varied while the pretiltangle is 3 degrees and azimuth φ=45 degrees. When no electric field isapplied (V =0 V), the graph is same as the curve 3 of FIG. 8 describedhereinabove. When an electric field is applied, a result is obtainedthat, as the zenithal angle α increases, the transmission decreases, andthe curve for V=3.0 V crosses in the proximity of the zenithal angleα=37 degrees, but the curve for V=3.5 V crosses in the proximity of thezenithal angle α=50 degrees, with the curve for V=0 V when no electricfield is applied (when the liquid crystal is in the initial orientationcondition), and thereafter the transmission factor and the brightnessare reversed. In other words, when the potential difference is 3.0 V,where the zenithal angle α is smaller than 37 degrees, the transmissionfactor is higher where a voltage is applied than where no voltage isapplied, but where the zenithal angle α exceeds 37 degrees, thetransmission factor is lower where a voltage is applied than where novoltage is applied. Accordingly, where the zenithal angle α exceeds 37degrees, a voltage applied portion becomes rather black while ano-voltage applied portion becomes rather white, and so-called gradationreversal wherein the black and white displays are reversed to ordinaryblack and white displays occurs. It is to be noted that, since thetransmission factors at a voltage applied portion and a no-voltageapplied portion are not much different from each other in the proximityof the zenithal angle α=37 degrees, the contrast is small and thedisplay image cannot be observed well. Similarly, where the potentialdifference is 3.5 V, gradation reversal wherein the transmission factorsbetween a voltage applied portion and a no-voltage applied portion arereversed to each other occurs around the zenithal angle α of 50 degrees.

The phenomenon of gradation reversal described above is observed alsowith actual devices. Although depending upon the relationship betweenthe pretilt angle of the liquid crystal and the directions of absorptionaxes of the polarizing plates, depending upon a direction in which theactive matrix liquid crystal display panel is viewed, gradation reversalsometimes occurs when the display panel is viewed from an angle of 40degrees.

In this manner, with the active matrix liquid crystal display apparatusof the IPS system which is constructed using a transverse electricfield, while a good display characteristic is obtained over a widerangle of visibility than that of the conventional TN system, there is aproblem in that, depending upon a direction in which the displayapparatus is viewed, gradation reversal occurs, and particularly where adisplay which includes much black is viewed from an oblique direction, agood image cannot be obtained.

As described above, when a substrate is viewed obliquely from adirection of, for example, 45 degrees with respect to the polarizationaxes of two polarizing plates which are in a positional relationship ofa cross nicol, a white floating phenomenon occurs because a phenomenonthat, at a portion at which no voltage is applied, transmission lightfrom one of the polarizing plates is absorbed but not completely by theother polarizing plate occurs. Further, since liquid crystal having arefractive index anisotropy is held between the two polarizing plates,the degree of the white floating phenomenon of the liquid crystaldisplay panel is not fixed-because light (linearly polarized light)having passed through one of the polarizing plates undergoes doublerefraction so that it is changed into elliptically polarized light,which enters the other polarizing plate. Where the directors of theliquid crystal on the plane of the substrate are oriented such thatprojections thereof extend in parallel to the polarization axis of oneof the polarizing plates and they define a fixed pretilt angle withrespect to the plane of the substrate as in an ordinary liquid crystaldisplay which makes use of a transverse electric field, as seen in FIG.8, the white floating intensity becomes very high depending upon therising direction of the liquid crystal. if the white floating isintensified in this manner, then gradation reversal sometimes occurs ata low zenithal angle as seen in FIG. 9.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems of the priorart described above, and it is a first object of the present inventionto provide an active matrix liquid crystal display apparatus of thetransverse electric field driven type which has a good displaycharacteristic free from coloring from whichever direction the displayapparatus is viewed.

It is a second object of the present invention to provide an activematrix liquid crystal display panel which suppresses rather whitecoloring of a black display portion without losing a good visibilityangle characteristic of a transverse electric field display and has agood display characteristic free from gradation reversal over a largervisibility angle range.

In order to attain the objects described above, according to an aspectof the present invention, there is provided an active matrix liquidcrystal display panel, comprising a first substrate on which a pluralityof color layers having transmission wavelengths different from eachother are provided in parallel to each other, a second substratedisposed in an opposing relationship to the first substrate with apredetermined clearance left from the first substrate for generating apredetermined electric field when a predetermined voltage is applied,and a liquid crystal layer formed from liquid crystal injected in a gapdefined by a surface of the first substrate adjacent the secondsubstrate and a surface of the second substrate adjacent the firstsubstrate, the electric field generated by the second substrate beingsubstantially parallel to the liquid crystal layer to control a display,the liquid crystal layer having a thickness which Varies depending uponthe transmission wavelengths of the color layers.

The liquid crystal layer may have a thickness which increases inproportion to one wavelength selected from a wavelength region in whichtransmission factors of the color layers are higher than 70% those atpeaks of transmission spectra of the color layers.

The second substrate may include a plurality of pixel electrodesprovided corresponding to the color layers, the predetermined voltagebeing applied to the pixel electrodes, and a plurality of opposingelectrodes provided in parallel to the pixel electrodes for each of thecolor layers for cooperating, when the voltage is applied to the pixelelectrodes, with the pixel electrodes to generate the electric fieldtherebetween, the pixel electrodes and the opposing electrodes beingspaced from each other by distances which are different for theindividual color layers.

The first substrate may have a protective layer provided on a surfacethereof adjacent the second substrate for preventing elusion ofimpurities from the color layers.

The reason why coloring occurs with a liquid crystal display apparatusof the transverse electric field driven type when the liquid crystaldisplay apparatus is viewed from an oblique direction arises from thefact that, when the factor f defined by the expression (2) variesdepending upon whether a ray comes in perpendicularly or from an obliquedirection, the manner of the variation is varied by λ.

A color liquid crystal display apparatus of a high quality with whichsuch coloring is called in question employs a color filter in almost allcases.

FIG. 14 is a diagram illustrating an example of transmission factorspectrum characteristics of color filters.

As seen from FIG. 14, the color filters selectively pass certain limitedwavelength regions corresponding to the three primary colors of R, G andB therethrough. The peaks of the transmission factor spectra of thecolor filters illustrated in FIG. 14 are 460 nm for blue, 540 nm forgreen and 640 nm for red, Further, the wavelength regions havingtransmission factors higher than 70% those at the peaks are 420 to 500nm for blue, 510 to 580 nm for green and 590 or more for red. In thosewavelength regions, since 70% or more of incoming light passes throughthe color filters, they have a significant influence on the displaycharacteristic.

Thus, if, taking a radiation spectrum of back light, a spectral luminousefficacy and so forth into consideration, a certain wavelength fromwithin the wavelength regions described above is selected as arepresentative and examined in regard to the transmission and so forthupon designing, then the values of the transmission factor and so forthfor an arbitrary wavelength in the wavelength regions becomesubstantially equal to each other within a range of conversion regardingthe transmission factor of the color filter.

Normally, since λ_(B)=460 nm, λ_(G)=550 nm and λ_(R)=610 nm for theblue, green and red color filters, respectively, are positionedsubstantially at the centers of the respective transmission wavelengthregions, they can be selected as the representative values.

Although the following description proceeds using the values mentionedabove as representative values, the specific values need not necessarilybe used as the representative values.

First, for the selected wavelengths λ_(R), λ_(G) and λ_(B), thethicknesses of the liquid crystal layer of pixels corresponding to thecolor filters are determined so as to satisfyd _(R)/λ_(R) =d _(G)/λ_(G) =d _(B)/λ_(B)   (10)

In this instance, the f factors of R, G and B when light comes in fromthe front are given byf _(R)=sin²(π·ΔndR _(eff)/λ_(R))   (11)f _(G)=sin²(π·ΔndR _(eff)/λ_(G))   (12)f _(B)=sin²(π·ΔndR _(eff)/λ_(B))   (13)where the effective thicknesses d_(Reff), d_(Geff) and d_(Beff) of theliquid crystal layer turned by the transverse electric field and thecell gaps d_(R), d_(G) and d_(B) have a relationship given by thefollowing expression:d _(Reff) /d _(R) =d _(Geff) /d _(G) =d _(Beff) /d _(B)   (14)Using the expressions (10) to (14),f_(R)=f_(G)=f_(B)   (15)is obtained.

On the other hand, if a substrate is viewed, in a white displaycondition, obliquely from a direction perpendicular to the directors asseen in FIG. 2, then the refractive index anisotropies Δn felt with theray do not vary, but only the optical path lengths L increase inaccordance with the following expressions:L _(R) =d _(Reff)/cos(θ′)   (16)L _(G) =d _(Geff)/cos(θ′)   (17)L _(B) =d _(Beff)/cos(θ′)   (18)where θ′ is the angle defined between the direction in which the lightadvances in the liquid crystal and a substrate normal, and strictlyspeaking, it is different for the individual colors where the refractiveindex has a wavelength dependency. However, since this wavelengthdependency is very small, it may be handled as being substantiallyfixed. Where the f factors when light comes in obliquely are representedby f′_(R), f′_(G) and f′_(B) for R, B and G, respectively, from thedefinition of f of the expression (2) and the expressions (10), (14),(16), (17) and (18),f′_(R)=f′_(G)=f′_(B)   (19)is obtained. Accordingly, as the inclination angle θ varies, althoughthe values of the factors f themselves vary, since they have quite samevalues also for different wavelengths, no coloring occurs.

While description is given above of the case wherein the direction inwhich a substrate is viewed is inclined to a direction perpendicular tothe directors of the liquid crystal, since the ratio between the opticalpath length and the wavelength is fixed in any other directionirrespective of the wavelength, the expression (19) stands in whicheverdirection the substrate is viewed and no coloring occurs.

This fact can be confirmed quantitatively by a simulation.

FIG. 15 is a diagram illustrating a relationship between the inclinationangle and the transmission factor in the active matrix liquid crystaldisplay apparatus of the present invention when light comes in, upondisplaying of white, in a direction perpendicular to the liquid crystaldirectors but oblique to the substrate, and FIG. 16 is a diagramillustrating a relationship between the inclination angle and thetransmission factor in the active matrix liquid crystal displayapparatus of the present invention when light comes in, upon displayingof white, in a direction same as the liquid crystal directors butoblique to the substrate. It is to be noted that, in FIGS. 15 and 16,assuming a case wherein white is displayed on a liquid crystal displayapparatus wherein the thickness of the liquid crystal layer is variedfor the individual colors of the color filters, the relationship betweenthe inclination angle θ of the ray and the transmission factor isobtained by calculation with lights having wavelengths of 610 nm, 550 nmand 460 nm representing the color filters of R, G and B (Red, Green andBlue), and the axis of abscissa represents the inclination angle of theincoming ray from the substrate normal and the axis of ordinaterepresents the transmission factor normalized with the fronttransmission factor. Here, the thicknesses of the liquid layercorresponding to R, G and B are 5.0 μm, 4.5 μm and 3.8 μm, respectively,while the intensity of the transverse electric field to be applied isset so as to increase in inverse proportion to the thickness of theliquid crystal layer so that the turning angles of the liquid crystallayer by the transverse electric field may be equal for R, G and B.

The azimuth in which the ray is inclined was taken, in FIG. 15, to adirection perpendicular to the turned liquid crystal directors, buttaken, in FIG. 16, to the same direction as the turned liquid crystaldirectors.

As apparently seen from FIGS. 15 and 16, as the inclination angle θvaries, the transmission factors vary, but they exhibit a same behaviorfor the wavelengths which represent the respective color filters.Accordingly, it was confirmed successfully also by the simulation thatno coloring occurs at all.

When the thickness of the liquid crystal layer is varied for theindividual colors of the corresponding color filters, the intensity ofthe transverse electric field necessary to turn the directors of theliquid crystal by a certain fixed angle increases in inverse proportionto the thickness of the liquid crystal layer. Accordingly, theintensities of the transverse electric fields to be applied in order toobtain a white display make the ratio of 3.8:4.5:5.0 for R, G and B.Therefore, when the distance between a pixel electrode and an opposingelectrode was set to 10 μm, the potential difference between the pixelelectrode and the opposing electrode in order to effect white displaywas 5.5 V for red, 6.0 V for green and 7.0 V for blue.

A system which provides voltages different for the individual colors inthis manner increases in complexity of circuitry and invites an increasein cost for a driving system. Therefore, the distance between a pixelelectrode and an opposing electrode is made different for the individualcolors such that it is 11 μm for red, 10 μm for green and 8.5 μm forblue so that a good white display can be obtained by applying 6 Vuniformly to pixels corresponding to all of the colors.

According to another aspect of the present invention, there is providedan active matrix liquid crystal display panel, comprising a plurality ofscanning lines and a plurality of signal lines disposed in anintersecting relationship with each other like gratings on one of a pairof transparent insulating substrates, a plurality of active elementsindividually provided in the proximity of intersecting points of thescanning lines and the signal lines, a plurality of pixel electrodesconnected to the active elements, a plurality of opposing electrodesdisposed corresponding to the pixel electrodes, a voltage being appliedbetween the pixel electrodes and the opposing electrodes, a liquidcrystal layer disposed between the one transparent insulating substrateand the other transparent insulating substrate, a pair of polarizingplates disposed on the outer sides of the transparent insulatingsubstrates, and a mechanism for controlling a display with an electricfield substantially parallel to the liquid crystal layer, and an opticalcompensation layer having a negative refractive index anisotropy in aone axis direction, a projection of the anisotropic axis of the opticalcompensation layer on a plane of one of the substrates being parallel toat least one of polarization axes of the two polarizing plates, theoptical compensation layer being disposed at least between the onetransparent insulating substrate and a corresponding one of thepolarizing plates.

Where the active matrix liquid crystal display panel is constructed suchthat, when the voltage between the pixel electrodes and the opposingelectrodes is 0 , angles formed by directors of liquid crystal moleculesin the liquid crystal layer with respect to a plane of the liquidcrystal layer are substantially uniform, and the refractive indexanisotropic axis of the optical compensation layer extends substantiallyin parallel to the directors, the accuracy in-compensation by theoptical compensation layer is improved.

Where a product Δn_(LC)·d_(LC) of a refractive index anisotropy Δn_(LC)and a layer thickness d_(LC) of the liquid crystal layer issubstantially equal to a product Δn_(F)·d_(F) of the refractive indexanisotropy Δn_(F) and a layer thickness d_(F) of the opticalcompensation layer, the compensation accuracy can be further improved.

Where a refractive index n_(LO) of the liquid crystal layer for ordinarylight and a refractive index n_(FO) of the optical compensation layerfor ordinary light are substantially equal to each other, the degree ofcompensation can be further improved.

Preferably, the active matrix liquid crystal display panel isconstructed such that, when a potential difference between the pixelelectrodes and the opposing electrodes is 0, projections of directors ofliquid crystal molecules in the liquid crystal layer on a plane of theliquid crystal layer are substantially parallel to each other and aprojection of the refractive index anisotropic axis of the opticalcompensation layer on the plane of the liquid crystal layer is parallelto the projections of the directors on the plane of the liquid crystallayer, and, where an angle of the refractive index anisotropic axis ofthe optical compensation layer with respect to the plane of the liquidcrystal layer is represented by θ_(F) and angles between the directorsand the plane of the liquid crystal layer on interfaces between theliquid crystal layer and the insulating substrates are represented by θ₁and θ₂, θ₁ and θ₂ being different from each other, the angle θ_(F)satisfies θ₁<θ_(F)<θ₂ or θ₂<θ_(F)<θ₁, and the refractive indexanisotropic axis of the optical compensation layer is parallel to thedirector of at least one of the liquid crystal molecules in the liquidcrystal layer.

Further preferably, the active matrix liquid crystal display panel isconstructed such that, when a potential difference between the pixelelectrodes and the opposing electrodes is 0, projections of directors ofliquid crystal molecules in the liquid crystal layer on a plane of theliquid crystal layer are substantially parallel to each other and aprojection of the refractive index anisotropic axis of the opticalcompensation layer on the plane of the liquid crystal layer is parallelto the projections of the directors on the plane of the liquid crystallayer, and, where an angle of the refractive index anisotropic axis ofthe optical compensation layer with respect to the plane of the liquidcrystal layer is represented by θ_(F) and angles between the directorsand the plane of the liquid crystal layer on interfaces between theliquid crystal layer and the insulating substrates are represented by θ₁and θ₂, and θ₂ being different from each other, the angle θ_(F) alwayssatisfies θ₁<θ_(F)<θ₂ or θ₂<θ_(F)<θ₁, and the angle θ_(F) varies in athicknesswise direction of the optical compensation layer in acorresponding relationship to a variation of the director in thethicknesswise direction of the liquid crystal layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a liquid crystal display system of thetransverse electric field driven type which exhibits a good displaycharacteristic;

FIGS. 2(a) and 2(b) are diagrammatic views illustrating passage of lightthrough liquid crystal when the light comes in obliquely, and whereinFIG. 2(a) is a view as viewed from an oblique direction to a substrateand FIG. 2(b) is a view as viewed in a direction parallel to thesubstrate;

FIG. 3 is a diagram illustrating a relationship between the inclinationangle and the transmission factor when light comes in, when white is tobe displayed, in a direction perpendicular to the liquid crystaldirectors and oblique to the substrate;

FIG. 4 is a diagram illustrating a relationship between the inclinationangle and the transmission factor when light comes in, when white is tobe displayed, in a direction same as that of the liquid crystaldirectors and oblique to the substrate;

FIG. 5 is a diagrammatic view showing a construction of a conventionalactive matrix liquid crystal display panel of the IPS system andillustrating the relationship between a polarization axis and thedirection of an electric field;

FIG. 6 is a diagram illustrating the relationship between the voltageand the transmission factor of a conventional active matrix liquidcrystal display panel of the TN system;

FIG. 7 is a diagram showing a polarization axis, the direction of a ray,the azimuth and the zenithal angle of the conventional active matrixliquid crystal display panel of the IPS system;

FIG. 8 is a diagram illustrating relationships between the zenithalangle and the transmission factor at various pretilt angles of theconventional active matrix liquid crystal display panel of the IPSsystem when no voltage is applied;

FIG. 9 is a diagram illustrating a relationship between the zenithalangle and the transmission factor of a conventional active matrix liquidcrystal display panel of the IPS system wherein the pretilt angle is 3degrees when a voltage is applied or no voltage is applied;

FIG. 10 is a diagram illustrating a relationship between the zenithalangle and the transmission factor of a conventional active matrix liquidcrystal display panel of the IPS system when a high voltage is applied;

FIGS. 11(a) and 11(b) are a sectional view and a plan view,respectively, showing a first embodiment of the active matrix liquidcrystal display apparatus of the present invention;

FIGS. 12(a) to 12(d) are views illustrating a method of controlling theliquid-crystal layer thickness, and wherein FIG. 12(a) is a sectionalview of a color filter provided with a spacer, FIG. 12(b) is a viewshowing the color filter combined with an active matrix substrate, FIG.12(c) is a sectional view where an overcoat layer is provided, and FIG.12(d) is a view where a granular spacer is provided;

FIGS. 13(a) and 13(b) are a sectional view and a plan view,respectively, showing a second embodiment of the active matrix liquidcrystal display apparatus of the present invention;

FIG. 14 is a diagram illustrating an example of transmission factorspectrum characteristics of color filters;

FIG. 15 is a diagram illustrating a relationship between the inclinationangle and the transmission factor when light comes into the activematrix liquid crystal display apparatus of the present invention, upondisplaying of white, from a direction perpendicular to the liquidcrystal directors and oblique to a substrate;

FIG. 16 is a diagram illustrating a relationship between the inclinationangle and the transmission factor when light comes into the activematrix liquid crystal display apparatus of the present invention, upondisplaying of white, from a direction same as that of the liquid crystaldirectors and oblique to a substrate;

FIG. 17 is a view showing an example of a construction of a spacerprovided in the active matrix liquid crystal display apparatus of thepresent invention;

FIG. 18 is a view showing a third embodiment of the active matrix liquidcrystal display apparatus of the present invention;

FIG. 19 is a sectional view of an active matrix liquid crystal displaypanel of the fourth embodiment of the present invention;

FIG. 20 is a plan view of an active matrix substrate in the fourthembodiment of the present invention;

FIG. 21 is a diagrammatic view illustrating a relationship among thepolarization axis, the liquid crystal directors and an the refractiveindex anisotropic axis of an optical compensation layer in the fourthembodiment of the present invention;

FIG. 22 is a diagram illustrating a relationship between the zenithalangle and the transmission factor of the fourth embodiment when novoltage is applied or a low voltage is applied;

FIG. 23 is a diagram illustrating a relationship between the zenithalangle and the transmission factor of the fourth embodiment when a highvoltage is applied;

FIG. 24 is a sectional view of an active matrix liquid crystal displaypanel of a fifth embodiment;

FIG. 25 is a sectional view of an active matrix liquid crystal displaypanel of a sixth embodiment;

FIG. 26 is a sectional view of an active matrix liquid crystal displaypanel of a sixth embodiment; and

FIG. 27 is a sectional view of an active matrix liquid crystal displaypanel of an eighth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

The first embodiment is described with reference to FIGS. 11(a) and11(b). Each pixel electrode 3 which forms a pixel is connected to thesource electrode of a thin film transistor which has a scanning line 16as a gate electrode thereof, and the drain electrode of the thin filmtransistor is connected to a signal line 1. The pixel electrode 3 has alongitudinal direction parallel to the signal line 1 and has an opposingelectrode 2 connected by an opposing electrode bus line 17.

A liquid crystal layer 4 is held between two glass substrates 10, andorientation films 23 are disposed on two substrate interfaces and areoriented uniformly in a rubbing direction 24 of FIG. 11(b) by rubbingthem in the same direction.

A pair of polarizing plates 5 disposed on the outer sides of the twoglass substrates 10 have polarization axes perpendicular to each other,and the polarization axis of one of the polarizing plates 5 coincideswith the initial orientation direction of the liquid crystal layer 4.

In the liquid crystal display apparatus of the transverse electric fieldtype having the construction described above, when the potentialdifference between the pixel electrode 3 and the opposing electrode 2 is0, black is displayed, and as the potential difference increases, theliquid crystal layer 4 is turned to cause double refraction thereby toraise the transmission factor. When the liquid crystal layer 4 is turnedapproximately by 45 degrees, the brightness exhibits its highest value.

A color filter is disposed on the opposing substrate and includes colorlayers 6, 7 and 8 for selectively passing the colors of red, green andblue therethrough, respectively, and a black matrix 9 provided toinhibit leakage of light from any other area than display areas in whicheffective display control is performed.

For each pixel, the cell thickness of the liquid crystal layer 4 isvaried in accordance with the color to be selected by the color filtersuch that it may be d_(R) for red, d_(G) for green and d_(B) for blue.In this instance, if the wavelength represented by a color of the colorfilter is se to λ_(R) for red, λ_(G) for green and λ_(B) for blue, thenthe layer thicknesses of the liquid crystal layer 4 corresponding to thecolors are determined so as to satisfy the following expression:d _(R)/λ_(R) =d _(G)/λ_(G) =d _(B)/λ_(B)   (20)

In order to vary the cell thickness of the liquid crystal layer 4 forthe individual colors of the color filter in this manner, such a colorfilter substrate provided with spacers 26 as shown in FIG. 12(a) and asubstrate having a TFT array formed thereon were combined in such amanner as seen in FIG. 12(b), and liquid crystal was filled between thetwo substrates to form the panel. It is to be noted that the spacers 26are formed on the black matrix 9 on the opposing substrate correspondingto crossing locations between the scanning lines 16 and the signal lines1. Consequently, in a condition wherein the two substrates are combinedin such a manner as shown in FIG. 12(b), the spacers 26 has such spacercontacting portions 14 as shown in FIG. 11(b).

In the thickness of the liquid crystal layer 4, while the transmissionfactors for the individual color layers of the color filter are keptequal to those of ordinary color filters which have such transmissionfactor characteristics as shown in FIG. 14, only the film thicknesses ofthe color layers were made different so as to satisfy the expression(20). Here, where the height of the spacers 26 is represented by t_(s),the thickness of the black matrix 9 is represented by t_(M) and thethicknesses of the scanning lines 16 and the signal lines 1 arerepresented by t_(v) and t_(H), respectively, the film thicknessest_(R), t_(G) and t_(B) of the color layers of R, G and B were controlledso as to satisfyt _(s) +t _(M) +t _(v) +t _(H) =t _(R) +d _(R) =t _(G) +d _(G) =t _(B)+d _(B)   (21)In this instance, the concentration of a pigment to be dispersed intoeach color layer is adjusted in accordance with the film thickness ofthe color layer.

It is also possible to form an overcoat layer on the color filter shownin FIG. 12(a) as seen in FIG. 12(c) in order to prevent elusion ofimpurities from the color layers.

Further, in order to dispose the two substrates parallelly in a spacedrelationship by a fixed distance from each other, granular spacers 25sprayed uniformly as seen in FIG. 12(_(d)) may be used in place of thespacers of the color filter with a spacer.

Further, while, in the present embodiment, the spacers 26 are providedat the crossing points between the scanning lines 16 and the signallines 1, they need not necessarily be provided at those locations, butif the substrates can be held in a fixedly spaced relationship from eachother, the spacers may be provided in any location in pixels.Preferably, however, the spacers 26 are provided at locations which doenot have an influence on a display and at which they are covered withthe black matrix 9. In this instance, since the thickness of the liquidcrystal layer formed with the spacers 26 is varied a little by thepattern of wiring lines or an insulation film, designing of the heightof the spacers 26 based on such variation is required.

Second Embodiment

FIGS. 13(a) and 13(b) are a sectional view and a plan view,respectively, showing a second embodiment of the active matrix liquidcrystal display apparatus of the present invention.

As shown in FIGS. 13(a) and 13(b), the present embodiment is quite sameas the first embodiment except that the distance between a pixelelectrode 3 and an opposing electrode 2 is different between a pixel 20corresponding to green and a pixel 21 corresponding to blue and thatnumerical aperture adjustment portions 22 are present.

Since the distance between a pixel electrode 3 and an opposing electrode2 is made different as seen in FIGS. 13(a) and 13(b), it can be realizedby applying the same potential that the transverse electric fieldintensity necessary to turn the liquid crystal layer 4 is differentamong the different colors, which arises from a variation of thethickness of the liquid crystal layer 4. Consequently, driving isfacilitated.

Further, the numerical aperture adjustment portion 22 is provided inorder to prevent the ratio of an effective display area held between apixel electrode 3 and an opposing electrode 2 occupied in one pixel,that is, the numerical aperture, from varying the distance between thepixel electrode 3 and the opposing electrode 2 by an opaque metal layerformed as the same layer as the opposing electrode 2 or the same layeras the pixel electrode 3. Since the distance between a pixel electrode 3and an opposing electrode 2 is largest with a pixel 19 corresponding tored and decreases in order of green and blue, the area of the numericalaperture adjustment portion 22 is largest with a pixel 19 correspondingto red, but is rather small with another pixel 20 corresponding togreen, and no numerical aperture adjustment portion 22 is provided for apixel 21 corresponding to blue.

The other construction is quite same as that of the first embodiment.

In the following, working examples of the embodiments described aboveare described in detail using detailed values including a method ofproducing the same.

First, a method of producing an active matrix substrate which is thesecond substrate is described.

As a metal layer from which signal lines 1, opposing electrodes 2 andopposing electrode bus lines 17 are to be formed, a Cr film is depositedwith 150 nm on a transparent glass substrate 10 and patterned.

Then, as a gate insulating film 11, a silicon nitride film of 400 nmthick, a non-doped amorphous silicon film of 350 nm thick and an n-typeamorphous silicon film of 30 nm thick are successively deposited.

Then, an n-type amorphous silicon film and a non-doped amorphous siliconlayer are formed in accordance with the pattern of island-shapedamorphous silicon 18.

Then, as a metal layer from which signal lines 1 and pixel electrodes 3are to be formed, a Cr film is deposited with the thickness of 150 nmand patterned.

Then, a protective insulating film 12 is formed, and the protectiveinsulating film 12 is removed at peripheral terminal locations thereofto complete a TFT array. In this instance, the patterns of the pixelelectrodes 3 and the opposing electrodes 2 are formed such that thedistances between them may be fixed to 10 μm with pixels correspondingto all colors as seen in FIGS. 11(a) and 11(b).

Now, a method of producing a color filter substrate as the firstsubstrate is described.

Photosensitive polymer of 0.1 μm thick containing carbon is formed on atransparent glass substrate and a black matrix layer 9 is provided usinga photo-lithography technique.

Then, photosensitive polymer containing a red pigment is formed on thesubstrate, and the photosensitive polymer formed in a region other thanthe region in which the red filter is formed is removed by aphoto-lithography technique to form the red filter 6.

Then, similar steps are performed to successively form the green filter7 and the blue filter 8.

The color filter produced in this manner had such transmission factorspectra as illustrated in FIG. 14.

Next, a method of forming the spacers is described.

FIG. 17 is a view showing an example of a construction of the spacersprovided in the active matrix liquid crystal display apparatus of thepresent invention.

As seen in FIG. 17, each spacer 26 is formed in a manner wherein allcolor layers are layered. Where a spacer 26 is formed in this manner,the height t_(S) of the spacer is given byt _(s) =t _(R) +t _(G) +t _(B)   (21)Where, taking the fact that the thicknesses of scanning lines 16 andsignal lines 1 are 0.15 μm and the relationship given by the expression(21) into consideration, the thicknesses t_(R), t_(G) and t_(B) of thecolor layers for R, G and B were set to 0.96 μm, 1.45 μm and 2.15 μm,respectively, the thicknesses of the liquid crystal layer 4 at pixelsfor the individual colors were 5.0 μm for red, 4.5 μm for green and 3.8μm for blue.

The thicknesses of the liquid crystal layer 4 mentioned above provide,where λ_(B)=460 nm, λ_(G)=550 nm and λ_(R)=610 nm are selected aswavelengths for representations of the color filters from thetransmission factor spectra of the color filter shown in FIG. 14, anequal ratio between the wavelength and the thickness of the liquidcrystal layer for the individual colors of the color filter.

On the color filter produced in this manner, an overcoat layer 13 wasformed with the thickness of 0.1 μm.

Orientation films 23 are applied to both of the active matrix substrateand the color filter substrate produced in such a manner as describedabove and then rubbed in the rubbing direction 24 shown in FIG. 11.Then, the two substrates are adhered to each other and secured atperipheries thereof to each other with a seal material, and then liquidcrystal is injected into and encapsulated in a space between thesubstrates to form a liquid crystal panel.

In the liquid crystal panel produced in such a manner as describedabove, since the ratios between the wavelengths representing the colorfilter and the thicknesses of the liquid layer are substantially equalfor the individual colors of the color filter, the principle describedhereinabove applies to this liquid crystal panel, and consequently, agood display characteristic free from coloring can be obtained.

While, in the working example described above, the overcoat layer isprovided on the color filter, it need not particularly be provided ifthe stability of the color layers is sufficiently high.

Further, while, in the working example described above, the spacers 26are formed by laying color layers of the color filter, a different layermay alternatively be formed to form the spacers 26 using aphoto-lithographic technique. Further, the two techniques may becombined to form the spacers 26 by layering the color layers and thedifferent layer.

Further, the spacers 26 need not be provided on the color filter, but asshown in FIG. 12(d), a granular spacer may be sprayed, upon combinationof liquid crystal, to control the thickness of the liquid crystal layer.

Furthermore, while, in the working example described above, thethickness of the liquid crystal layer is varied corresponding to theindividual colors by making the thicknesses of the color layers of thecolor filter different from each other, the thicknesses of the liquidcrystal layer corresponding to the individual colors may be controlledby layering dielectric layers different from the color layers on theindividual layers while varying the thicknesses of the dielectric layersas seen in FIG. 18.

FIG. 17 is a view showing a further embodiment of the active matrixliquid crystal display apparatus of the present invention.

Further, by using the same production procedure while varying the shapesof the patterns of pixel electrodes 3 and opposing electrodes 2 in sucha manner as seen in FIG. 13, a liquid crystal display panel formed fromthe second embodiment was obtained.

In this instance, the distance between a pixel electrode 3 and anopposing electrode 2 was set to 11 μm for pixels corresponding to red,10 μm for pixels corresponding to green and 8.5 μm for pixelscorresponding to blue. Further, in order to prevent the area of adisplay region held between a pixel electrode 3 and an opposingelectrode 2 from being varied for the individual colors, numericalaperture adjustment portions 22 were provided for pixels 19corresponding to red and pixels 20 corresponding to green. Consequently,while the difference between a pixel potential and an opposing potentialnecessary to obtain the highest brightness was different, in the workingexample of the first embodiment, for the individual colors, in a workingexample of the second embodiment, by application of 6.0 V, the highestbrightness was obtained with pixels corresponding to all of the colors.Besides, since the equal numerical aperture was obtained for all of thecolors without the necessity for a special countermeasure in structure,a good white characteristic was obtained successfully.

Since the present invention is constructed in such a manner as describedabove, it exhibits the following effects.

In the active matrix liquid crystal display apparatus as set forth inclaims 1 and 2, since the liquid crystal layer has a thickness whichvaries depending upon transmission wavelengths of the color layers, theactive matrix liquid crystal display apparatus can provide a very gooddisplay free from any coloring from whichever direction it is viewed.

In the active matrix liquid crystal display apparatus set forth in claim3, since the distance between a pixel electrode and an opposingelectrode is set different for the individual color layers, an equalvoltage can be applied to the pixel electrodes corresponding to theindividual color layers to achieve the effect described above, andconsequently, driving is facilitated.

In the active matrix liquid crystal display apparatus set forth in claim4, since the protective layer is provided on the surface of the firstsubstrate adjacent the second substrate, elusion of impurities from thecolor layers can be prevented.

FIG. 19 is a sectional view showing a principal portion of the fourthembodiment of the active matrix liquid crystal display panel of thepresent invention, and FIG. 20 is a plan view of an active matrixsubstrate A of the active matrix liquid crystal display panel. Theactive matrix substrate A of the IPS system (in the present embodiment,on the light incoming side) is described. A plurality of opposingelectrodes 2 connected to each other by an opposing electrode bus line17 and a scanning line 16 are formed on a glass substrate (transparentinsulating substrate) 10, and a gate insulating film 11 is formed insuch a manner as to cover over the opposing electrodes 2 and thescanning line 16. Further, island-shaped amorphous silicon 18 whichmakes part of a thin film transistor (hereinafter referred to simply as“TFT”), which is an active element, a plurality of pixel electrodes 3and a signal line 1 are formed on the gate insulating film 11. The pixelelectrodes 3 and the signal line 1 extend in parallel to the opposingelectrodes 2. Further, a protective insulating film 12 and anorientation film 23 are formed in a layered relationship. The sourceelectrode of the TFT is connected to the pixel electrodes 3 while thedrain electrode is connected to the signal line 1, and the scanning line16 serves as the gate electrode of the TFT. The active matrix substrateA of the IPS system having the TFT is formed in this manner. It is to benoted that details of the method of production are hereinafterdescribed.

A color filter substrate C (in the present embodiment, on the lightoutgoing side) includes an orientation film 56 same as that on theactive matrix substrate A side and provided on one of the two surfacesof another glass substrate (transparent insulating substrate) 10, and anoptical compensation layer 35 formed from a plastic film and provided onthe other surface of the glass substrate 10.

The active matrix substrate A and the color filter substrate C aredisposed such that the orientation films thereof are opposed to eachother, and a pair of polarizing plates are disposed on the outer sidesof the two substrates while a liquid crystal layer 4 having a positiverefractive index anisotropy is provided between the orientation films 23of the two substrates. It is to be noted that the polarizing plate onthe light incoming side serves as a polarizing plate 5 and thepolarizing plate on the light outgoing side serves as an analyzer 34.

FIG. 21 illustrates the relationship among a polarization direction 48of the polarizer, a polarization direction 46 of the analyzer, adirector direction 51 of a liquid crystal molecule, a direction 47 ofthe refractive index anisotropy axis of the optical compensation layer,a substrate normal 49, a longitudinal direction 50 of the electrodes anda direction 52 of an electric field. The substrate normal 49, thelongitudinal direction 50 of the electrodes and the direction 52 of theelectric field expend perpendicularly to each other. Each broken line inFIG. 21 represents the polarization direction 48 of the polarizer. Thepolarization direction 48 extends in parallel to a substrate plane 53and defines a fixed angle with respect to the longitudinal direction 50of the electrodes. The polarization direction 46 of the analyzer isperpendicular to the polarization direction 48 of the polarizer.

Liquid crystal molecules are oriented uniformly by the orientation film23, and the directors 51 (longitudinal directions) thereof are inclinedby a fixed angle (pretilt angle) with respect to the substrate plane 53.The pretilt angle normally ranges approximately from 1 to 10 degrees.Projections of the directors 51 of the liquid crystal molecules on thesubstrate plane 53 extend in parallel to the polarization direction 48of the polarizer, and the refractive index anisotropic axis 47 of theoptical compensation layer extends in parallel to the directors 51. Thepolarization direction 46 of the analyzer extends perpendicularly to thepolarization direction 48 of the polarizer and in parallel to thesubstrate plane 53.

Some of conventional active matrix liquid crystal display panels of thetransverse electric field type are controlled based on the followingtheory. In particular, where the potential difference between a pixelelectrode and an opposing electrode is 0 (when no electric field isapplied), light is absorbed by the polarizer and the analyzer and blackis displayed. However, if an electric field is applied, then thedirectors are turned, and as the potential difference increases, thedirectors 51 are further turned. Consequently, components which are notabsorbed by the analyzer increase in a ray which has passed through theliquid crystal layer and the transmission factor increases, approachinga white display. Then, when the directors 51 are turned approximately by45 degrees, the transmission factor (brightness) exhibits a maximumvalue.

Conventionally, however, even if control is performed based on thistheory, it sometimes occurs that the display does not look well. Asdescribed above, when a substrate is viewed obliquely, principally fromthe fact that linearly polarized light after passing through thepolarizing plates 5 undergoes, when it passes through the liquid crystallayer 4, a retardation so that it is converted into ellipticallypolarized light, even when no electric field is applied and the liquidcrystal molecules are not turned, light sometimes comes into theanalyzer 34 from the liquid crystal layer 4 while it includes polarizedlight components which cannot be absorbed by the analyzer 34. Accordingto a result of detailed numerical calculation with the relationshipbetween the direction of the pretilt angle and the direction of the raytaken into consideration, when viewed from a direction 54 (refer to FIG.21), the transmission factor is very high comparing with that where theliquid crystal layer 4 is not present and only the polarizing plates ofa cross nicol are viewed from the same direction 54. In other words,where black is to be displayed, it looks rather white and thisdeteriorates the display quality.

Therefore, in the present invention, the optical compensation layer 35is provided. In the present embodiment, the optical compensation layer35 which has a negative one axis refractive index anisotropy is providedbetween the glass substrate 10 and the analyzer 34, and the refractiveindex anisotropic axis 47 thereof extends in parallel to the directors51 of the liquid crystal while the optical main axis in the liquidcrystal layer 4 and the optical main axis in the optical compensationlayer 35 extend in a substantially same direction. When light passesthrough the liquid crystal layer 4, it undergoes distortion of thepolarization plane thereof by a retardation, and the polarization planedistorted in this manner is compensated for by the optical compensationlayer 35 so that the polarization condition of the light approaches thepolarization condition (linear polarization) at the time immediatelyafter the light passes through the polarizer 5. Then, after the lightpasses through the optical compensation layer 35, it is absorbed by theanalyzer 34 so that black is displayed. In this manner, the presentinvention exhibits an effect in that white floating in a black displaycan be suppressed by canceling a retardation which occurs in the liquidcrystal layer 4 when black is to be displayed by means of the opticalcompensation layer 35 irrespective of the incoming direction of the ray.Besides, little influence is had on any other visibility anglecharacteristic than this. Accordingly, a liquid crystal display panelwhich has a very wide visibility angle characteristic can be obtained.

As described above, since the direction 47 of the optical axis(refractive index anisotropic axis) of the optical compensation layer 35is the same as the direction (direction of directors) 51 of the opticalaxis of the liquid crystal layer 4, at whichever angle light comes in,the optical main axis of the light when the light passes through theliquid crystal layer 4 and the optical main axis of the light when thelight passes through the optical compensation layer 35 are substantiallysame as each other, and the liquid crystal layer 4 having a positiverefractive index anisotropy and the optical compensation layer 35 havinga negative refractive index anisotropy can be canceled effectively.Further, even if the optical compensation layer 35 which has arefractive index anisotropic axis in this direction is present, thetransmission factor when the liquid crystal display panel is viewed fromthe front is not varied by it at all and also the visibility anglecharacteristics of white and half tones other than the black level arevaried little. Accordingly, white floating of a black display can beprevented efficiently and gradation reversal can be prevented, and abetter visibility angle characteristic can be achieved.

The distortion of the polarization plane of the light when the lightpasses through the liquid crystal layer 4 is composed of a retardationwhich increases in proportion to the product of the refractive indexdifference between the optical main axes and the optical path lengths.In order to correct the distortion, a retardation in the oppositedirection should be applied by the optical compensation layer 35. If therefraction indices of the liquid crystal layer 4 and the opticalcompensation layer 35 with regard to ordinary light are substantiallyequal to each other, then the ratios between the layer thicknesses andthe optical path lengths are substantially equal to each other. Further,since the anisotropic axes of the refractive indices are common to eachother and also the-main axes upon passage of the ray are substantiallysame as each other, also the refractive index difference between theoptical main axes and the refractive index anisotropies of theindividual layers increase in proportion to each other. From theforegoing, by making the product ΔnL_(C)·dL_(C) of the refractive indexanisotropy ΔnL_(C) and the liquid crystal layer thickness dL_(C) of theliquid crystal layer 4 and the product Δn_(F)·d_(F) of the refractiveindex anisotropy Δn_(F) and the layer thickness d_(F) of the opticalcompensation layer 35 substantially coincide with other, the distortion(retardation) of the polarization plane produced in the liquid crystallayer can be corrected substantially fully by the optical compensationlayer, white floating can be suppressed to a level substantially equalto that obtained where only the cross Nicol is used.

It is to be noted that, as described above, in order to achieve morecomplete compensation, the refractive index of the liquid crystal layer4 for ordinary light and the refractive index of the opticalcompensation layer 35 for ordinary light are preferably set equal toeach other. Where the refractive indices of them are different from eachother, a ray passes in finely different directions through the layers,resulting in fine differences of the directions of the optical mainaxes, the refractive index differences on the main axes and the opticalpath lengths, and consequently, complete compensation cannot beachieved. However, if the refractive indices of them are made coincidewith each other, then the optical main axes coincide completely witheach other, and compensation of retardations of the liquid crystal layer4 and the optical compensation layer 35 can be achieved more completely.

A relationship of the zenithal angle 55 and the transmission factor inthe active matrix liquid crystal display panel when a substrate isactually viewed from a direction of the azimuth of 45 degrees withreference to the direction of the polarization axis 48 of the polarizeras shown in FIG. 21 is illustrated in FIG. 22. While, where the opticalcompensation layer 35 is absent, the transmission factor exhibits areversal at the small zenithal angle 55 of approximately 35 degrees asseen in FIG. 9, by employing the optical compensation layer 35, thezenithal angle 55 at which the transmission factor exhibits a reversalcan be driven to a range higher by 10 degrees or more, and also thebrightness when a transmission factor reversal occurs can be suppressedto a considerably low level.

A decrease of the white brightness of an electric field applied portionof the active matrix liquid crystal display panel when it is viewed froman oblique direction where the optical compensation layer 35 is absentis illustrated in FIG. 23, and a decrease of the white brightness of anelectric field applied portion of the active matrix liquid crystaldisplay panel when it is viewed from an oblique direction where theoptical compensation layer 35 is provided is illustrated in FIG. 10.From FIGS. 23 and 10, it can be seen that, with the liquid crystaldisplay panel which includes the optical compensation layer 35, thedecrease of the white brightness is suppressed smaller than that withthe panel which does not include an optical compensation layer, anddeterioration of the display quality is suppressed low at both of ablack display portion and a white display portion by the action of theoptical compensation layer 35.

An example of a method of producing a liquid crystal display panelhaving such a construction as described above is described in detail.

First, a method of producing the active matrix substrate A is described.

As a metal layer from which the scanning lines 16, opposing electrodes 2and opposing electrode bus lines 17 are to be produced, a Cr film islayered with 150 nm on a transparent glass substrate and then patterned.Further, as the gate insulating film 11, a silicon nitride film of 400nm thick, a non-doped amorphous silicon film of 350 nm thick and ann-type amorphous silicon film of 30 nm thick are successively layered.Thereafter, the n-type amorphous silicon layer and the non-dopedamorphous silicon layer are patterned to form island-shaped amorphoussilicon 18. Then, as a metal layer from which the signal lines 1 and thepixel electrodes 3 are to be formed, a Cr film is layered with 150 nmand then patterned. Further, the protective insulating film 12 is formedand then removed at peripheral terminal portions thereof, therebycompleting a TFT.

To the active matrix substrate A produced in such a manner as describedabove and a color filter substrate C, the orientation films 23 and 56are applied, respectively. The orientation film 23 on the active matrixsubstrate side is rubbed in the direction 24 in FIG. 19, and theorientation film 56 on the color filter substrate side is rubbed in theopposite direction to the direction 24 in FIG. 24. The two substratesare disposed such that the two orientation films 23 oppose each otherand are secured to each other at peripheral portions thereof by a sealmember (not shown). Thereafter, liquid crystal is injected into andenclosed in a gap between the two orientation films to provide theliquid crystal layer 4. It is to be noted that the liquid crystaldirectors 51 are oriented substantially in a fixed direction in theliquid crystal layer 4 by the orientation films 23 and 56. The pretiltangle between the liquid crystal directors 51 and the substrate plane 53in the present embodiment is 3 degrees. The refractive index of theinjected liquid crystal for ordinary light is no=1.476 and therefractive index anisotropy is Δn=0.067, and in order to optimize thebrightness of a white display and the color reproduction property, thecell gap was set to 4.5 μm.

Further, a plastic film to serve as the optical compensation layer 35 isapplied to the outer side of the color filter substrate. The opticalcompensation layer 35 has a negative one axial refractive index factoranisotropy, and the refractive index anisotropic axis extends in adirection parallel to the initial orientation direction of the liquidcrystal directors 51, that is, in a direction in which it defines 3degrees with respect to the plane of the substrate. The productΔn_(F)·d_(F) of the refractive index anisotropy Δn_(F) and the layerthickness d_(F) of the optical compensation layer was set equal to theproduct of the refractive index anisotropy and the layer thickness ofthe liquid crystal layer and 302 nm.

Two polarizing plates are applied such that the active matrix substrateA and the color filter substrate C are held between them. In thisinstance, the polarization axis 48 of the polarizer (light incoming sidepolarizing plate) 5 extends in parallel to the rubbing direction 24while the polarization axis of the analyzer (light outgoing sidepolarizing plate) 34 extends in a direction perpendicular to thedirection of the polarization axis 48.

The liquid crystal display panel produced in this manner was drivenactually. It was revealed that a good display characteristic wherein theblack level was stabilized over a visibility angle range wider than everand little gradation reversal was found was obtained successfully, andthe liquid crystal display panel was able to be used over a visibilityangle range of 50 degrees in the upward and downward directions and theleftward and rightward directions.

Next, a fifth embodiment of the present invention is described in detailwith reference to the drawings.

The active matrix liquid crystal display panel of the present embodimenthas an almost same construction and is produced by an almost sameproduction method as the fourth embodiment, but is different from thefourth embodiment in the orientation directions of two orientation films57 and 58 and the angle defined between the directors of liquidcrystal-molecule and the plane of a substrate.

FIG. 24 is a sectional view showing a section of the liquid crystaldisplay panel taken along a plane including the polarization axis of thepolarizer and a substrate normal line in order to show a direction 59 ofthe directors of liquid crystal molecules and a direction 61 of therefractive index anisotropic axis of an optical compensation layer 60.Here, the signal lines 1, scanning lines 16, island-shaped amorphoussilicon 18, pixel electrodes 3, opposing electrodes, polarizationdirection 48 of the polarizer, polarization direction 76 of the analyzerand so forth have same constructions as those of the fourth embodiment(refer to FIGS. 19 to 21).

The orientation films 57 and 58 are subject to orientation processing(rubbing) in the same direction (same direction as the direction 24 ofFIG. 20). The direction 59 of the directors liquid crystal moleculesvary in a liquid crystal layer 62. While projections of the directors ofall liquid crystal molecules on the plane of the substrate extend in thesame direction parallel to the polarization direction 48 of thepolarizer, the angle defined between the directors 59 of liquid crystalmolecules and the plane of the substrate is different between that onthe light incoming side substrate interface and that on the lightoutgoing side substrate interface. Where the angles are represented byθ₁ and θ₂, respectively, the angle θL_(C) of the directors with respectto the plane of the substrate continuously varies between the twointerfaces and is distributed so as to minimize the strain energy.

The optical compensation layer 60 formed from a plastic film applied tothe outer side of the color filter substrate C has a negative one axisrefractive index anisotropy, and the direction 61 of the refractiveindex anisotropic axis is set such that a projection thereof on theplane of the substrate extends in parallel to projections of thepolarization axis 48 of the polarizer and the directors 59 of liquidcrystal molecules on the plane of the substrate. Further, the angleθ_(F) defined between the anisotropic axis 61 of the opticalcompensation layer and the plane of the substrate is uniform in theinside of the layer and θ₂<θ_(F)<θ₁ and 0.45 degrees in the presentembodiment. It is to be noted that, otherwise if θ₁<θ₂, then the angleθ_(F) is set so as to satisfy θ₁<θ_(F)<θ₂. The material of the liquidcrystal and the cell thickness are same as those of the fourthembodiment, and the product Δn_(F)·d_(F) between the refractive indexanisotropy Δn_(F) and the layer thickness d_(F) of the opticalcompensation layer is equal to the product of the refractive indexanisotropy and the layer thickness of the liquid crystal layer 62 and302 nm in the present embodiment.

The polarization axis 48 of the polarizer 5 from between the twopolarizing plates applied to the outer sides of the liquid crystaldisplay panel extends in parallel to the rubbing direction 24, and thepolarization axis of the analyzer 34 extends in a directionperpendicular to the rubbing direction 24 (refer to FIG. 20).

In the present embodiment, an optimum value of θ_(F) can be determinedby simulation or experiment although this is not very simple because theoptical main axis in the liquid crystal layer varies in thethicknesswise direction. Convenietly, the optimum value of θ_(F) may begiven as θ_(F)=(θ₁+θ₂)/2. Where the optimum value of θ_(F) is used, theretardation of the liquid crystal layer 62 and the retardation of theoptical compensation layer 60 when black is to be displayed cancel eachother considerably well, and white floating in a black display can besuppressed to such a degree as that of a cross nicol.

The active matrix liquid crystal display panel which was produced insuch a manner as described above had a very wide visibility anglecharacteristic similarly as in the fourth embodiment.

It is to be noted that, in order to obtain a good black display,projections of the directors of liquid crystal molecules on the plane ofthe substrate are normally held substantially in coincidence with thepolarization axis of a polarizing plate on one side. Then, also aprojection of the refractive index anisotropic axis of the opticalcompensation layer 60 on the plane of the substrate is set to the samedirection. Further, the angle θ_(F) defined between the refractive indexanisotropic axis of the optical compensation layer and the plane of thesubstrate can be set to a suitable position between θ₁ and θ₂ so thatwhite floating can be suppressed efficiently.

Next, a sixth embodiment of the present invention is described in detailwith reference to the drawings.

The active matrix liquid crystal display panel of the present embodimenthas an almost same construction and is produced by an almost sameproduction method as the fifth embodiment, but is different from thefifth embodiment in the angle defined between an optical compensationlayer 63 and the plane of a substrate.

FIG. 25 is a sectional view showing a section of the liquid crystaldisplay panel taken along a plane including the polarization axis of thepolarizer 5 and a substrate normal line in order to show a direction 64of the directors of liquid crystal molecules and a direction 65 of therefractive index anisotropic axis of the optical compensation layer 63.Here, the signal lines 1, scanning lines 16, island-shaped amorphoussilicon 18, pixel electrodes 3, opposing electrodes 2, polarizationdirection 48 of the polarizer, polarization direction 46 of the analyzerand so forth have same constructions as those of the fourth embodiment(refer to FIGS. 19 to 21).

Similarly as in the fifth embodiment, the two orientation films 57 and58 are subject to orientation processing (rubbing) in the same direction(same direction as the direction 24 of FIG. 20). The direction of thedirectors 64 of liquid crystal molecules varies in a liquid crystallayer 66. While projections of the directors of all liquid crystalmolecules on the plane of the substrate extend in the same directionparallel to the polarization direction 48 of the polarizer 5, the angledefined between the directors of liquid crystal molecules and the planeof the substrate is different between that on the light incoming sidesubstrate interface and that on the light outgoing side substrateinterface. Where the angles are represented by θ₁ and θ₂, respectively,the angle θ_(LC)(z) of the directors with respect to the plane of thesubstrate continuously varies between the two interfaces and isdistributed so as to minimize the strain energy.

The polarization axis of the light incoming side one (polarizer) 5 ofthe two polarizing plates adhered in such a manner that the twosubstrates are held between them extends in parallel to the rubbingdirection 24 (refer to FIG. 20), and the polarization axis of the lightoutgoing side polarizing plate (analyzer) 34 extends in a directionperpendicular to the rubbing direction 24 (refer to FIG. 20).

The optical compensation layer 63 has a negative one axis refractiveindex anisotropy and is set such that a projection of the refractiveindex anisotropic axis on the plane of the substrate always extends inparallel to projections of the polarization direction 48 of thepolarizer and the directors of liquid crystal molecules on the plane ofthe substrates. Further, as seen in FIG. 25, the angle defined betweenthe refractive index anisotropic axis 65 of the optical compensationlayer and the plane of the substrate varies in the inside of the layer,and this angle is a function θ_(F)(ξ) of the coordinate ξ in thedepthwise direction. θ_(LC)(z) and θ_(F)(ξ) are set so as to satisfy thefollowing relations:θF(ξ)=θ_(LC)(z)   (22)ξ=z·d _(F) /d _(LC)   (23)where d_(F) is the thickness of the optical compensation layer, d_(LC)the thickness of the liquid crystal layer, and θ_(LC)(z) the angledefined between the directors of liquid crystal molecules at theposition of the depth z in the liquid crystal layer 66 and the plane ofthe substrate.

θL_(C)(z) is distributed in accordance with the following expression:$\begin{matrix}{{\theta_{LC}(z)} = {\theta_{1} - {\theta_{2} \cdot \frac{z}{d_{LC}}} + \theta_{2}}} & (24)\end{matrix}$

If the direction of the refractive index anisotropic axis of the opticalcompensation layer is varied so as to satisfy the relationship givenabove, slab surfaces corresponding to each other compensate for eachother, and accordingly, the efficiency is high.

It is to be noted that the material of the liquid crystal and the cellthickness are same as those of the fourth embodiment, and the productΔn_(F)·d_(F) between the refractive index anisotropy Δn_(F) and thelayer thickness d_(F) of the optical compensation layer is equal to theproduct of the refractive index anisotropy and the layer thickness ofthe liquid crystal layer and 302 nm in the present embodiment.

In the present embodiment, since the optical main axis in the liquidcrystal layer varies in the thicknesswise direction z, by varyingθ_(F)(ξ) in accordance with the variation, a further better visibilityangle characteristic of a black display can be obtained comparing withthe fifth embodiment.

In the three embodiments described above, an optical compensation layeris provided between the analyzer 34 and a glass substrate 10. However,an optical compensation layer 67 may otherwise be held between thepolarizer 5 and a glass substrate 10 as seen in FIG. 26. In thisinstance, almost similar effects can be obtained if the constructionsuch as the direction of the refractive index anisotropic axis of theoptical compensation layer 67 is the same as that in one of the threeembodiments described above.

Meanwhile, a further construction may alternatively be employed wherein,as shown in FIG. 27, optical compensation layers 68 and 69 are providedboth between the analyzer 34 and one of the glass substrates 10 andbetween the polarizer 5 and the other glass substrate 10. If thedirections of the anisotropic axes of them are set parallel to eachother and the sum of products of Δn and d of the two opticalcompensation layers is set equal to Δn_(LC)·d_(LC) of the liquid crystallayer 4, then almost complete compensation can be achieved.

Further, while, in the embodiments described above, projections of thepolarization axis of the polarizer and the directors of liquid crystalmolecules on the plane of a substrate are set parallel to each other,similar effects can be obtained even if projections of the polarizationaxis of the analyzer and the directors of the liquid crystal on theplane of the substrate are set parallel to each other and thepolarization axis of the polarizer is set perpendicular to them.

As described above, according to the present invention, since an opticalcompensation layer having a negative one axis refractive indexanisotropy in an active matrix liquid crystal display panel, aretardation produced in a liquid crystal layer can be canceled tosuppress white floating of a black display portion and gradationreversal can be suppressed significantly, and an image characteristicwhich is good in a wider visibility angle can be obtained.

1.-15. (canceled)
 16. An active matrix liquid crystal display panel,comprising: a plurality of scanning lines and a plurality of signallines disposed in an intersecting relationship with each other likegratings on one of a pair of transparent insulating substrates, aplurality of active elements individually provided in the proximity ofintersecting points of said scanning lines and said signal lines, aplurality of pixel electrodes connected to said active elements, aplurality of opposing electrodes disposed corresponding to said pixelelectrodes, a voltage being applied between said pixel electrodes andsaid opposing electrodes, a liquid crystal layer disposed between theone transparent insulating substrate and the other transparentinsulating substrate, a pair of polarizing plates disposed on the outersides of said transparent insulating substrates, and a mechanism forcontrolling a display with an electric field substantially parallel tosaid liquid crystal layer; and an optical compensation layer having anegative refractive index anisotropy in a one axis direction, aprojection of the anisotropic axis of said optical compensation layer ona plane of one of said substrates being parallel to at least one ofpolarization axes of said two polarizing plates, said opticalcompensation layer being disposed at least between the one transparentinsulating substrate and a corresponding one of said polarizing plates.17. An active matrix liquid crystal display panel as claimed in claim16, wherein, when the voltage between said pixel electrodes and saidopposing electrodes is 0, angles formed by directors of liquid crystalmolecules in said liquid crystal layer with respect to a plane of saidliquid crystal layer are substantially uniform, and the refractive indexanisotropic axis of said optical compensation layer extendssubstantially in parallel to said directors.
 18. An active matrix liquidcrystal display panel as claimed in claim 16, wherein a product ΔnLC·dLCof a refractive index anisotropy ΔnLC and a layer thickness dLC of saidliquid crystal layer is substantially equal to a product ΔnF·dF of therefractive index anisotropy ≢nF and a layer thickness dF of said opticalcompensation layer.
 19. An active matrix liquid crystal display panel asclaimed in claim 17, wherein a product of a ΔnLC·dLC of a refractiveindex anisotropy ΔnLC and a layer thickness dLC of said liquid crystallayer is substantially equal to a product ΔnF·dF of the refractive indexanisotropy ΔnF and a layer thickness dF of said optical compensationlayer.
 20. An active matrix liquid crystal display panel as claimed inclaim 16, wherein the liquid crystal layer is an in-plane switch (IPS).